Milk of livestock animals, of which typical examples are cow, sheep, and goat, is not sterile, and may be contaminated with certain microorganisms due to diseases or environment. In particular, it is known that animals with a disease caused by infection of a microorganism often discharge a lot of the microorganisms into milk. Typical diseases of livestock animals caused by infection of a microorganism include mastitis.
Mastitis is inflammation of the laticifer system or milk gland tissue, and it is caused largely by invasion, ecesis, and proliferation of a microorganism in the udder. Although many kinds of animals contract mastitis, it is said that, especially concerning cow's mastitis in dairy cows, 15 to 40% of the whole dairy cows contract mastitis, and thus it is one of the extremely important diseases for dairy farmers. If a dairy cow contracts mastitis, not only the milk synthesis function is inhibited to result in reduction of lactation amount, or even stop of lactation as the case may be, but also enormous economical losses such as cost of medical treatment and penalty concerning milk price are imposed on dairy farmers. Furthermore, it also increases the labor of dairy farmers, since, for example, milking of teats suffering from mastitis must be separately performed for preventing infection.
Mastitis is caused by infection of various microorganisms, but antibiotics that exhibits efficacy against mastitis may differ depending on type of causative microorganism, and certain types of microorganisms have different characteristics concerning, for example, transmission to other teats or individuals, or post-infectious handling thereof differs. Therefore, it is extremely important to quickly and conveniently identify the causative microorganism existing in milk.
As methods for identifying a causative microorganism of an infectious disease, there are known cultivation-based identification method, gene-based identification method, and antigen-antibody reaction-based identification method. Although the current mainstream of the method for identifying a causative microorganism of mastitis of livestock animals is the cultivation-based identification method, the operation thereof is complicated, and in addition, it has a problem of requiring several days for obtaining results. There has also been reported an identification method based on detection of a specific gene by a gene amplification method (PCR method) (Non-patent document 1). Although results can be obtained in about one day by this method, it still has a problem of requiring special instruments and maneuvers. In recent years, there has been developed an apparatus for detecting Staphylococcus aureus, which is one of the causative microorganisms of mastitis, or Escherichia coli by surface plasmon resonance (SPR) on the basis of an antigen-antibody reaction (Non-patent document 2). For this apparatus, an antibody against an antigen specific to each bacterium is prepared, and this antibody is fixed on an SPR chip and used. Although this method enables quick detection of a specific causative microorganism, it requires a special apparatus, and therefore it is difficult to perform measurement by this method on practical dairy spots etc.
Simple measurement devices which enable quick and convenient measurement in the home or clinic for biosamples such as blood and urine utilizing immunochromatography (immunochromatographic device) have widely been used (for example, refer to Patent document 1). The method performed in such devices uses a test strip comprising test paper containing a first antibody specific to a target substance of measurement (antigen) and labeled with coloring particles such as gold colloid, and a porous membrane on which a second antibody for capturing the target substance of measurement is immobilized, which test paper and porous membrane are linked together. If a test sample containing a target substance of measurement is dropped onto the strip, the target substance of measurement binds with the antibody labeled with coloring particles and/or the antibody immobilized on the porous membrane, resulting a visually distinguishable line or the like on the membrane. Therefore, by confirming presence or absence of such a line or the like, presence or absence of the substance to be measured can be detected.
Although Patent document 1 teaches that the aforementioned method can be applied to, besides blood (whole blood), plasma, serum, urine, saliva, sputum, sweat, and so forth, it does not teach application of the aforementioned method to milk of livestock animals, and does not teach nor suggest the problems encountered at the time of such application at all. Further, although methods for applying an immunochromatographic method to biosamples such as blood (whole blood) as a test sample are also disclosed in Patent documents 2 to 4, these patent documents do not teach nor suggest application of immunochromatographic method to milk of livestock animals, either.
For a simple measurement device that enables quick and convenient measurement for milk of livestock animals as a test sample by using an immunochromatographic method at practical dairy spots, Patent document 5 proposes a method of using the immunochromatographic method for milk as a test sample for the purpose of inspecting causative microorganisms of mastitis of livestock animals. This patent document teaches that milk fat globules and casein contained in milk inhibit detection by the immunochromatographic method, and it is preferred that they are removed in advance before the test. Although this patent document discloses a method of removing milk fat globules and casein by leaving milk standing, separating a cream layer, and performing a treatment with a surfactant, it does not teach removal of milk fat globules with a filter.
Further, although Patent documents 2 to 4 teach techniques of removing contaminants with a physical filter or a membrane showing chemical affinity for use in applying immunochromatographic method, the methods described in these references are methods of applying the immunochromatographic method to biosamples such as whole blood as described above, and they are not methods of applying it to milk of livestock animals. Patent documents 1 to 4 do not teach nor suggest the problems encountered at the time of applying the immunochromatographic method to milk of livestock animals, and do not teach any means for solving the problems at all.